1. Field of the Invention
The invention generally relates to detectors of electromagnetic radiation. More specifically, the invention relates to a radiation sensor providing attitude data for alignment of the sensor with respect to an impinging beam of radiation. Most specifically, the invention relates to a radiation sensor using a variable spacing aperture plate and a two-dimensional photodetector array to provide data for alignment of the sensor with respect to a ground-based laser illuminating the sensor.
2. Description of Prior Art
Detection of radiant energy in such a manner as to provide attitude/range information relating the sensor and the energy source is important in many areas and a variety of competing techniques exist within the prior art to provide such information.
One technique, typically found with tracking telescopes, uses a telescope with a photodetector or photodetector array at it's focal plane. The position of the image of the source, measured by the photodetector, provides the desired information. Generally, such lens systems are complex, relatively heavy, and have resolution limited to the quality of the lenses used. Further, such systems have angular range limited to that of the objective lens used. Lens having greater resolution and angular range may be used but only with much greater cost and complexity.
Another technique within prior art is that of using an array of highly directional receivers consisting of a dedicated photodetector and an associated electronic circuit for each discrete receiver. Such systems are typically very complex and expensive and resolution depends on the number and size of each receiver and the size of the overall arrays. Further, response time and resolution may be sacrificed due to vibration and speed restrictions inherent to mechanical systems.
A third technique is that of transforming the incoming radiation into a line image having a radiant power distribution along it's length. The angular orientation of the radiation source may then be determined by illuminating a linear array of discrete photodetectors and having the strongest signal from a given detector represent the angle of the source. Alternatively, the line image may be scanned by a single photodetector and the intensity variation along the line image may be peculiar to the angular incidence of the illuminating radiation.
The present invention avoids the use of lenses and their attendant sensitivity to environmental degradation. Further, greatly increased resolution and angular range is achieved with the present invention at far lower cost than equivalent systems using lenses. Finally, the present invention provides greater resolution and angular range with less size and complexity than conventional dedicated array or mechanically-scanned array systems. While conventional systems might provide equivalent resolution or angular range to the present invention, none can provide the combination achievable with the present invention.